1. Field of the Invention
The present invention relates to a recombinant influenza neuraminidase, an expression vector with which the recombinant neuraminidase can be expressed in host cells, methods for producing and purifying recombinant neuraminidase, vaccines against influenza and the use of recombinant neuraminidase according to the invention.
2. Description of the Related Art
Influenza A and B virus epidemics cause considerable discomfort to those affected and have a great influence a on social and economic life. They cause a significant mortality rate in older people and in patients with chronic illnesses. Since their introduction during the 1940s, inactivated vaccines based on virus material cultured in chicken eggs have been found to be clearly effective against influenza infection and have resulted in a significant fall in the mortality rate of high-risk populations.
The influenza viruses are unique among the viruses of the bronchial tubes because they undergo a significant antigenic variation (so-called “drift”) in their two surface antigens, that is, the hemagglutinin (HA) and the neuraminidase (NA).
In addition, influenza A in particular can escape the prevalent immunity due to the phenomenon of “shift”. Appearing herein in the human virus is an NA gene which comes from the animal reservoir of influenza genes. In 1957 the NA1-type virus prevalent up to that time was thus replaced by a new NA2-type virus. Since 1977 the NA1-type viruses have also returned to the human population. The present vaccines must therefore preferably be aimed against both NA1 and NA2-type viruses.
NA catalyses the removal of terminal sialic acid residues of glycosyl groups whereby potential receptors for HA are destroyed (Gottschalk, 1957; Burnet and Stone, 1947). It is assumed that NA is essential in preventing virus aggregation and in an efficient spreading from cell to cell (Colman, and Ward, 1985).
Each NA molecule (Mr=240,000) has a toadstool-like structure which consists of four identical polypeptide chains built up of two dimers which are linked to disulphide bridges and in turn held together by non-covalent bonds (Bucher and Kilbourne, 1972; Laver and Valentine, 1969; Varghese et al., 1983; Ward et al., 1983). Otherwise than HA, NA is anchored in the lipid membrane by a non-spliced, NA-terminal, lipophilic sequence (Fields et al., 1983; Block et al., 1982), the so-called membrane anchor. The greatest part of the total structure protrudes above the membrane and for s there a distal, box-shaped “head” area localised on top of an elongate “stalk” region (Wrigley et al., 1973′). Inside the head each monomer has its own catalytic site and contains at least four NA-linked glycosyl groups (Colman et al., 1983; Ward et al., 1982). The presence of O-glycosylation has not yet been demonstrated up to the present time.
On account of their external localization the HA and NA antigens represent the most important viral target structures for the host immune system. Of antibodies which bind specifically to HA it is thought that they neutralise the viral infectivity, probably by blocking the early steps of infection (Hirst, 1942; Kida et al., 1983). NA-specific antibodies normally do not prevent the initial infection of a target cell (Jahiel and Kilbourne, 1966; Kilbourne et al., 1968; Johanssen et al., 1988) but precisely the spread of the virus. In addition, due to competition mechanisms, the immunologic response to NA appears to be partly suppressed in favour of the more frequently occurring HA antigen (Johanssen et al., 1987, Kilbourne, 1976). As net result the effect of NA immunity is generally overshadowed by the neutralising HA antibodies. For this reason the attention of vaccine designers has been focussed for a long time almost exclusively on HA.
A number of experimental observations indicate however that NA is indeed capable of playing a significant part in the build-up of protective immunity to influenza (Schulman et al., 1968; Johansen and Kilbourne, 1990; Johansen et al., 1993). Fundamental studies into the immunogenic potential of NA necessitate the availability of very pure antigens in sufficient quantities and with the correct three-dimensional conformation. Up until now NA has been prepared by treating viral envelopes with detergents (Gallagher et al., 1984; Kilbourne et al., 1968) or by proteolytic cleavage of the protein head, often by means of pronase (Seto et al., 1966; Rott et al., 1974), followed by purifying of the NA. Although to some extent usable, these methods have considerable limitations in respect of yield and purity.